。
Basically, I don't trust him.
Literal
Essentially I [topic-は] him [possessive-の] thing [object-を] trust-is-not-[ていない].
要するに ('in short / essentially / to sum up') is a summarizing discourse marker, slightly more formal than 要は. 信用していない ('don't trust,' with ~ている for state) is the natural way to express a stance — 信用しない would be more like a one-time decision not to trust. Dropping は after 彼のこと is casual spoken rhythm.